A procedure and a device for operating a combustion engine are known from DE 199 03 439 A1, in whose exhaust gas area a SCR-catalyzer (selective catalytic reduction) is arranged, which reduces the nitrous gases that are contained in the exhaust gas of the combustion engine with a reagent to nitrogen.
The reagent ammoniac is for example used thereby as a reagent, which can be won from a urea-water-solution or a urea-formic-acid-water-solution as a precursor of the reagent. The metering of the reagent or of the precursor of the reagent has to take place very precisely. A metering that is too low causes that nitrous gases in the SCR-catalyzer cannot be sufficiently reduced anymore. A metering that is too high causes a so-called reagent slip, which can cause an unnecessary high reagent consumption on the one hand and, depending on the kind of the reagent, an unpleasant odor nuisance on the other hand.
A procedure for operating a reagent metering valve is known from DE 10 2006 044 080 A1, at which a reagent is metered by an electromagnetically controllable valve. The metering valve is impinged with a pulse-width modulated metering signal, which determines the metering of a precursor of a reagent that has to be introduced into the exhaust gas area of a combustion engine. The opening time of the metering valve is limited to a minimal opening time, at which the metering valve is completely opened.
The opening time is thereby determined in such a way that a fine spray mist is created. Thereby a crystallizing of the reagent is avoided. Hereby an exact metering is possible; moreover it is worked against a blocking of the metering valve. The minimal opening time depends on several parameters, as for example the temperature and the operating voltage of the metering valve magnet as well as on the reagent pressure.
DE 10 2007 044 807.6 describes a procedure for operating a metering valve and a device for implementing the procedure, at which the electromagnetically operated metering valve is impinged with a pulse-width modulated metering signal. Hereby a clock signal is provided whose cycle duration is shorter than the cycle duration of the pulse-width modulated metering signal. The cycle duration of the pulse-width modulated metering device is determined to multiple cycle durations of the clock signal depending on a metering demand signal.
The problem with this procedure is that in some cases the metered amount does not correspond exactly with the requirements, which means with the desired metering amount. Furthermore the metering amount does not only depend on the opening time of the valve, but is also determined by several other factors. The amount is mostly determined by the reagent pressure.
The invention is therefore based on the task to improve a procedure for operating an electromagnetically operated metering valve in such a way that a mostly precise metering of the reagent is enabled, whereby in particular influences of the reagent pressure are considered.